In the prior art, as the joining member for connecting the ink accommodating member and ballpoint pen tip provided in a ballpoint pen, joining members such as those disclosed in Patent References 1 and 2 have been known. In Patent Reference 1, a breechblock (joining member) which is an injection-molded part of polybutylene terephthalate (hereafter “PBT”) resin is proposed. According to this Patent Reference 1, by using the above-described PBT, there is no cracking or splitting due to stresses in pressure-fit connection, nor is there deformation in elevated-temperature environments, and penetration of gas molecules is suppressed.
Further, a joining member (pen tip holder) disclosed in Patent Reference 2 is molded from polyethylene naphthalate resin. According to this Patent Reference 2, the above joining member of course has transparency, but also has excellent heat resistance and shock resistance, adequately prevents ink evaporation and leakage, and gives no cause for concern of any detriment to functions as a writing instrument or application instrument.    [Patent Reference 1] Japanese Unexamined Patent Publication No. 2005-288863    [Patent Reference 2] Japanese Unexamined Patent Publication No. 9-85160